1. Field of the Invention
This invention relates to a stepping motor and, more particularly, relates to an improvement of a stepping motor wherein stator windings thereof can be used in a three-phase driving and a two-phase driving.
2. Description of the Prior Art
In the most of conventional hybrid type stepping motors shown in FIGS. 4 and 5 or the conventional permanent magnet type stepping motors, two-phase windings are used.
FIG. 4 shows a vertically sectional side view of a conventional hybrid type stepping motor 10.
FIG. 5 is a vertically sectional front view, taken along lines 5--5 of FIG. 4.
In FIGS. 4 and 5, a reference numeral 1 denotes a stator housing, 2 denotes a stator core, 3-1-3-8 denote stator windings, and 4 and 4' denote front and back brackets, respectively. The stator core 2 has main poles 2-1-2-8. Reference numeral 2-10 denotes pole teeth formed on the inner peripheral surface of each of the main poles 2-1-2-8.
Stator windings 3-1-3-8 are wound around the main poles 2-1-2-8, respectively. The stator core 2 and the stator windings 3-1-3-8 form a stator S.
In FIG. 4, reference numerals 5 and 5' denote bearings, 6 denotes a rotor shaft, 7 and 8 denote rotary magnetic poles, 7-10 and 8-10 in FIG. 5 denote pole teeth formed on the outer peripheral surfaces of the rotor magnetic poles 7 and 8, respectively. The pole teeth 7-10 and 8-10 are deviated in the circumferential direction of said rotor magnetic poles by 1/2 pitch of the pole teeth from each other.
A rotor R is formed of said rotary shaft 6, rotary magnetic poles 7 and 8 and a permanent magnet 9 held between said two rotary magnetic poles 7 and 8.
Such rotor R is called a hybrid type rotor and forms a part of conventional two-phase hybrid type stepping motor 10.
One of the important factors showing the abilities of the stepping motor is a step angle .theta. s which is normally determined by a phase number P of stator windings or a pole tooth number Zt formed on each rotor magnetic pole and expressed by EQU .theta.s=180.degree./(P.multidot.Zt) (1)
The step angle .theta. s expressed by the formula (1) is proper to the stepping motor, and becomes small if the phase number P or the pole tooth number Zt becomes large.
The step angle .theta. s of a conventional two-phase stepping motor can be expressed by .theta. s=180.degree./(2.multidot.Zt) and, accordingly, it is very difficult technically to manufacture a two-phase stepping motor having a very small step angle, because if the pole tooth number Zt is larger, the width of the rotor pole tooth becomes smaller.
Further, the two-phase hybrid type stepping motor makes large oscillation and noise.
Such defects can be obviated if the number of the stator main poles is increased to enhance the magnetic balance. However, no two-phase stepping motor of the main magnetic pole number other than eight or the multiple thereof can be obtained.
In the conventional stepping motor having the hybrid type rotor shown in FIGS. 4 and 5, if the number of the stator main poles is 6, the rotor tooth number becomes 6n+5 or 6n+1, where n is an integer and .gtoreq.1, so that the rotor tooth number becomes an odd number. Accordingly, the one-phase windings are wound around the main poles separated by 180.degree. from each other, so that if the one-phase windings are excited the two rotor magnetic poles holding the permanent magnet therebetween are magnetized in the opposite polarities. As a result, one of the rotor magnetic poles is pulled in the upward direction, whereas the other of the rotor magnetic poles is pulled in the downward direction, and, accordingly, the rotor shaft is received a force couple normal to the axis of the rotor shaft, so that oscillation and noise are generated.
Further, in the conventional permanent magnet type stepping motor, the main magnetic poles on which one-phase windings are wound are magnetized in the same polarity, as shown in U.S. Pat. No. 5,386,161, so that a sufficient magnetic path cannot be formed in case that the windings of one phase are excited, because no different polarities are formed on the main poles on which the one-phase windings are wound.
Accordingly, it is necessary to magnetize in the opposite polarity the main poles on which another one-phase windings are wound, and to form a magnetic path between the main poles of opposite polarities.
In this manner, a two-phase or multiple-phase excitation has to be adopted in practice.
If the phase number P of the stator windings is increased to four or five, however, the number of switches in the driving circuit is also increased.
This result in an expensive and complicated circuit.